1. Field of Invention
This invention relates to the field of Low Light Level Imaging for Night Vision Applications.
2. Related Art
Photocathode based image intensifier tubes, Electron Bombarded CCDs (EBCCD), and Electron Bombarded Active Pixel Sensors (EBAPS) are well known for the purpose of acquiring images under low light level illumination conditions. EBAPS are described in U.S. Pat. No. 6,285,018 B1. EBCCDs are described in U.S. Pat. No. 4,687,922. These photocathode based devices employ a wide array of photocathode materials ranging from traditional multi-alkali or bi-alkali photocathodes used in so called Generation-II image intensifiers to modern negative electron affinity (NEA) III-V semiconductor photocathodes using GaAs, GaAs1-xPx (lattice constant and bandgap determined by value of x, 0≦x≧1), and related III-V semiconductor materials. Image intensifiers employing GaAs photocathodes are used in Generation III image intensifiers that are widely used in Night Vision Goggles as described in U.S. Pat. No. 6,597,112. An InGaAs NEA photocathode for use in image intensifiers is described in U.S. Pat. No. 6,121,612. EBAPS cameras using GaAs photocathodes are widely used for low light level imaging cameras and digital night vision systems.
More recently Transferred Electron (TE) photocathodes using an InGaAs light absorbing layer lattice matched to InP (InGaAs composition is In0.53Ga0.47As for InP lattice matched InGaAs) and an InP emission layer have been employed in EBAPS for active systems utilizing laser illumination at 1.06 microns or in the 1.5 to 1.6 micron wavelength range. TE photocathodes are described in U.S. Pat. Nos. 5,047,821 and 5,576,559. In these systems the laser is pulsed and the photocathode-to-anode high voltage is pulsed in a timing relationship with the laser to enable imaging of illuminated objects at a predetermined range from the system employing the laser and EBAPS camera.
Solid State focal plane arrays are also employed in low light level imaging systems targeted for Night Vision applications. Exemplary Solid State (semiconductor) imaging sensors include silicon CMOS imaging arrays, silicon charge coupled devices (CCD), hybridized photodiode or avalanche photodiode arrays where the light sensing photodiodes are bonded to a Read Out Integrated Circuit (ROIC), and germanium photodetectors integrated with a silicon readout circuit. In this disclosure the term photodiode will be defined to include photodiode, avalanche photodiode, and photodetectors directly integrated with the ROIC.
The hybridized photodiode array enables use of semiconductors other than silicon for the light sensing element. Semiconductors used for the hybridized photodiode array include Hg1-xCdxTe of varying compositions determined by value of x between 0 and 1, InSb, and InGaAs among other materials known in the art. These semiconductor materials have sensitivity in other spectral regions than silicon. Typically these materials have a long wavelength cutoff in the SWIR or Infra-Red portion of the electromagnetic spectrum with long wavelength cutoffs ranging from 1.7 μm to 12 μm. The long wavelength cutoff is determined by the energy bandgap of the material. The energy bandgap is a function of the material composition for the ternary and quaternary semiconductor alloys. These semiconductor materials may have short wave cutoffs extending into the visible (0.4-0.7 micron wavelength) or Near Infra-Red (0.7-1 micron wavelength) portions of the spectrum. InGaAs photodiode array image sensors are described in U.S. Pat. Nos. 6,852,976 B2 and 6,573,581 B1.
Monolithic image sensors utilizing photosensitive elements comprised of germanium that is directly integrated with the silicon readout circuit are disclosed in U.S. Pat. Nos. 7,453,129 and 7,651,880 B2. These image sensors employ germanium as the optical absorbing material and have a long wavelength cutoff of 1.6 μm.
Night Vision sensors are constrained to use the available light at night. FIG. 1 shows the relative nighttime photon flux versus wavelength on moonlit and moonless nights as measured by Vatsia over the 0.47 to 1.93 micron spectral range. The variation in available light at particular wavelength bands is due to atmospheric absorption over specific regions due to water and other atmospheric gases and due to the emission spectrum of optical sources at night including moonlight, starlight, and emission from the upper atmosphere. At wavelengths greater than 2 microns thermal radiation due to blackbody emission of objects at ambient temperature becomes significant relative to other sources of available light at night and one moves into the realm of thermal imaging. The object of this invention is to provide improved night time imaging performance using reflected ambient light, not thermal imaging using blackbody radiation.
Examination of FIG. 1 shows that substantially more light is available on moonless nights at longer wavelengths. The low amount of light available on moonless nights provides the greatest challenge to Night Vision image sensors. Night Vision device performance is limited by the amount of available light on moonless nights. Due to the increased photon flux at wavelengths above 1.5 μm, designers of night vision image sensors have focused on developing sensors that extend sensitivity to these higher wavelengths, i.e., to include detection of photons in the 1.4-1.9 μm range. An example of this is disclosed in U.S. Pat. No. 7,608,825 B2 where the long wavelength cutoff of the semiconductor optical absorber layer is extended from 1.7 μm for In0.53Ga0.47As to wavelengths as long at 3.0 μm by epitaxial growth of GaInNAsP, GaInNAsSb or GaInNAs semiconductor layers of varying composition of the Group III and Group V constituents of the quaternary or quinary semiconductor lattice matched to an InP substrate.
High performance sensors have other requirements for good night vision performance in addition to spectral response range. Two fundamental requirements are high quantum efficiency (photon detection efficiency) over the sensitive wavelength range with low associated dark noise. Dark noise is dominated by thermally generated dark current in a high performance sensor. Dark current magnitude is a critical performance parameter as fundamentally the dark noise associated with the dark current needs to be less than the photogenerated current to obtain imaging performance at a given light level. The combination of these requirements on the photosensing element limits the choice of semiconductor materials for the photocathode or photodiode and specifies image sensor operating temperature.
InGaAs lattice matched to an InP substrate has been used in the past to meet these requirements for the photosensing material. The InGaAs composition that results in the same lattice constant as InP is In0.53Ga0.47As this composition of InGaAs is also denoted by the term “lattice matched InGaAs or InGaAs lattice matched to InP” in this disclosure. InGaAs lattice matched to InP is sensitive to 1.7 micron and shorter wavelength light. The short wavelength cutoff ranges from 0.4 to 0.95 micron depending upon the specific structure of the photocathode or photodiode. This wavelength range captures substantially more light than the commonly used GaAs, Generation-III, photocathode that is sensitive over the 0.4-0.9 micron Visible Near Infra-Red (VNIR) wavelength band. Use of InGaAs lattice matched to InP minimizes material defects in the epitaxially grown InGaAs layer. This minimizes dark current typically generated by defects in the epitaxial layer.
There is considerable interest in extending the spectral response of manportable night vision systems into the Short Wave Infra-Red (SWIR) spectral band (0.95-1.7 μm for an In0.53Ga0.47As photodiode focal plane array or photocathode with an integral InP substrate). For these applications performance, size, weight, and power are all critical requirements. Today hybridized photodiode/ROIC focal plane arrays (FPA) consisting of an array of In0.53Ga0.47As photodiodes bump bonded to an ROIC are the state-of-the-art. However these FPAs have performance, size, and power constraints that limit their applicability to manportable night vision systems.
System size, weight, unit cost, and power consumption are critical night vision system metrics. Size and performance are impacted by the relatively large pixel size (15 μm for state-of-the-art, developmental, FPAs and 25 μm for commercially available product) of the available SWIR FPAs. The minimum pixel size is limited by the bump bonding technology used to attach the photodiode array to the ROIC. This restricts overall sensor format to SXGA (1280×1024) and lower array sizes to maintain an acceptable overall FPA size. Larger FPAs would increase system size and cost to unacceptable levels for manportable applications.
The smaller array size limits system resolution and object detection range for the desired large field of view systems used for manportable night vision. A typical image intensifier tube based night vision goggle has a 40° Field of View and the equivalent resolution of a modern image intensifier is approximately 6 to 9 megapixels. One object of this invention is to increase the array size of a SWIR image sensor relative to the prior art for use as a night vision image sensor.
Power is impacted from the requirement for some level of cooling for nighttime SWIR imaging applications other than laser spot detection. Cooling is required as the level of dark current emitted by either an In1-xGaxAs or other semiconductor used for imaging in the SWIR band or at longer wavelengths is too high at room temperature (20°-25° C.) to enable low light level imaging on moonless nights. The provision of cooling for portable applications places a high demand on batteries, thereby reducing the useful operational time of the device before batteries are emptied.
A SWIR EBCCD can be achieved by replacing the GaAs photocathode with an In0.53Ga0.47As based photocathode. This has been demonstrated by Intevac for Laser Illuminated Viewing and Ranging (LIVAR®) applications (V. Aebi and P. Vallianos, “Laser-illuminated viewing provides long-range detail,” LASER FOCUS WORLD, September 2000). These applications utilize a pulsed eyesafe laser illuminator typically operating at 1.57 μm. The present LIVAR camera utilizes an InGaAs photocathode used in an EBAPS with a CMOS anode array.
In LIVAR applications the photocathode is gated on for very short periods (typically in the 100 ns to 2 μs time range). This low duty cycle enables operation at ambient temperature (<40 degrees C.) without performance being photocathode dark current limited. InGaAs lattice matched to InP and used in a TE photocathode configuration is a good choice for this application with 20 degree C. emitted dark current in the 10-40 nA/cm^2 range and good sensitivity in the 1.5-1.6 micron spectral range for compatibility with eyesafe laser illuminators.
Passive night time imaging requires continuous integration of the light by the image sensor to maximize light collection. Under these conditions substantial cooling is required of the In0.53Ga0.47As TE photocathode to reduce dark current to an acceptable level to achieve signal limited performance at night (typically cooling to −40 degrees C. is required). This level of cooling substantially increases system size and power requirements making the approach unsuitable for battery operated night vision applications.
Other focal plane arrays used for night vision applications also require cooling to reduce focal plane dark current to levels where it does not limit performance on moonless nights. This includes InGaAs FPAs where dark current limits performance at operating temperatures above about 10° C. and monolithic Ge photodetector arrays directly integrated with a ROIC that require cooling to the −70° C. range. The smaller bandgap semiconductors such as InSb or various alloys of Hg1-xCdxTe also require cooling to temperatures substantially below 20° C. to reduce dark current. These cooling requirements limit application of these SWIR image sensors to battery operated applications due to high power requirements for focal plane cooling.